This invention relates to chrominance signal reproducing apparatus for use in a reproducing system such as a video tape recoder (hereinafter, referred to as VTR), or the like.
FIG. 1 shows a typical example of a conventional chrominance signal reproducing apparatus for color VTR.
The operation of the apparatus of FIG. 1 upon reproduction will be described. Upon reproduction, a reproduced signal output from a pre-amplifier (not shown) is applied via a terminal 1 to a low-pass filter (hereinafter referred to as LPF) 5, where a low-frequency converted signal is separated from an FM modulated signal. The signal from the LPF 5 is applied to a chrominance signal gain automatic control (hereinafter abbreviated ACC) circuit 6. The low-frequency converted chrominance signal, the signal amplitude of which is made constant at the ACC circuit 6, is applied to a main frequency converter 7 together with a frequency converting signal from a band-pass filter 14. The frequency converted components from the frequency converter 7 are applied to a first band-pass filter (hereinafter, abbreviated BPF) 8 with its center frequency being the difference-component frequency, where the difference component frequency is separated as a chrominance signal f'sc before low-frequency conversion.
This chrominance signal f'sc is applied via an output terminal 4 to a mixer (not shown) for mixing with the reproduced luminance signal and so on, to appear as a reproduced video signal. However, the reproduced carrier chrominance signal includes a time-base variation .DELTA.f due to the irregular tape drive speed of the tape driving system. Thus, at the reproducing step, in case of converting the low-frequency converted carrier chrominance signal to the carrier chrominance signal before low-frequency conversion, an automatic frequency control circuit (hereinafter, abbreviated AFC circuit) and an automatic phase control circuit (hereinafter, abbreviated APC circuit) are provided as correction circuits for compensating for the time-base variation. These correction circuits control two variable frequency oscillators for allowing the output of a sub converter to be applied as a frequency converting signal to the main converter 7 and thereby allow the sub converter to generate the frequency converting signal for canceling the time-base variation. The AFC circuit is a correction circuit using the phase information of horizontal synchronizing signals, and comprises a phase comparator 15, a 160 f.sub.H VCO 16 (f.sub.H : horizontal synchronizing frequency) which is a voltage-controlled oscillator for generating 160 f.sub.H as a reference frequency, a 1/4-frequency divider 17 and a 1/40-frequency divider 18. The phase comparator 15 compares the phase of the reproduced horizontal synchronizing signal f'.sub.H from an input terminal 2 and the phase of 1/160 the output of the VCO 16 or f.sub.H thereby to control the VCO 16 by the application of the error voltage corresponding to the phase difference thereto. The output of the VCO 16 is divided in frequency by 4 at the frequency divider 17 and then applied to a phase shifter 19 as a signal having a frequency of 40 f.sub.H +.DELTA.f' (.DELTA.f': time-base variation). The phase shifter 19 is responsive to a signal obtained by reproducing a vertical synchronizing signal recorded on a magnetic tape from a terminal 3 to advance the phase of the 40 f.sub.H +.DELTA.f' signal by 90.degree. at each H (H: horizontal scanning period) when one of two tracks recorded on the tape is reproduced, or delay it by 90.degree. at each H when the other track is reproduced, in the NTSC system. In the PAL system, the phase shifter 19 is responsive thereto to delay the phase of the 40 f.sub.H +.DELTA.f' signal by 90.degree. at each scanning period when one of two tracks is reproduced. Thus, the phase-shifted signal is then applied to the sub converter 13.
The APC circuit comprises a burst gate circuit 9, a crystal oscillator 11 for producing a signal of the same frequency as the carrier chrominance frequency f.sub.sc, a phase comparator 10, and a VCO 12. The burst gate 9 is responsive to the reproduced synchronizing signal applied via a low-pass filter (LPF) 20 to extract the color burst signal from the reproduced chrominance signal which has been reconverted to the chrominance signal before the low-frequency conversion. Then, the phase comparator 10 compares the reference signal (chrominance carrier frequency) from the crystal oscillator 11 and the color burst signal to supply an error voltage corresponding to the phase difference for correcting the time base variation in the carrier chrominance signal, to the VCO 12 thereby controlling the oscillation frequency of the VCO 12.
Consequently, the VCO 12 produces a signal having a frequency of f.sub.sc +.DELTA.f" (.DELTA.f": time-base variation) and supplies it to the sub converter 13. The sub converter 13 produces the difference and sum components of the outputs from the VCO 12 and phase shifter 19, of which the sum component (f.sub.sc +40f.sub.H +.DELTA.f, where .DELTA.f=.DELTA.f'+.DELTA.f") is taken out by the band-pass filter (BPF) 14 and then applied to the main converter 7 as the frequency converting signal. Therefore, the difference component of the outputs of the main converter 7, or the output of the BPF 8 has the time-base variation .DELTA.f removed. However, with this arrangement, the two correcting circuits (AFC and APC circuits) always operate separately. In other words, in the AFC circuit, the output of the VCO 16 is always controlled while in the APC circuit the output of the VCO 12 is always controlled. Since the two circuits have different response speeds to the phase variation, the control system sometimes becomes unstable, losing correct and stable chrominance reproduction. Moreover, in the long time mode, in the luminance signal processing system, a complex signal processing is performed for improvement of S/N ratio and so on and also nonlinear processing is carried out. Thus, the waveform of the video signal may cause the waveform of the horizontal synchronizing signal in the reproduced luminance signal to change, changing the phase of the horizontal synchronizing signal separated from the reproduced luminance signal. The result is that this phase change is also transmitted to the 160 f.sub.H VCO, and then via the sub and main converters to the chrominance signal. This phase change is generally not absorbed enough in the APC system loop, and eventually the luminance signal adds the phase error to the chrominance signal to spoil correct color reproduction.